The zone refining of gallium has been practised in the past, but it is noted in many references that the process is difficult to execute because of the low melting temperature of gallium, its high degree of expansion upon solidifying and its relatively high thermal conductivity.
Pfann, in U.S. Pat. No. 2,739 088, mentions the zone refining of gallium in a closed ring that rotates in a horizontal or inclined plane. Detwiler and Fox (Trans. AIME 203, p. 205, January 1955 J. of Metals) zone refined a gallium ingot in a closed Pyrex tube sliding inside heating and cooling coils, using the reciprocating method. Richards (Nature 177, (4500), p. 182, Jan. 28, 1956) stated that zone refining of gallium was not effective, and he purified gallium indirectly by zone refining gallium chloride and reducing the purified chloride to metal. Liu Min-Chih (Acta Phys. Sinica 15, (7), 389, 1959) produced 69 gallium by zone refining in a horizontal rotating spiral coil made of a plastic material. The coil is cooled at its lower portion and heated at its top portion creating moving solid and liquid zones. Gallium has also been zone refined by moving an ingot of gallium over a plate having alternating cooling and heating zones, or by moving spaced heaters over the ingot which is cooled from below (Acta Chim. Acad. Sci. Hung., 24, 466-472, 1960). Zone refining of gallium has been enhanced by electromagnetic stirring (Chem. Abs., volume 70, 6194h), the use of seed crystals (Trans. Met. Soc. AIME 221, 889, Aug. 1961), vacuum melting (Chem. Abs., volume 80, 98667x), and a heat exchanger that supercools the melt during solidification (Chem. Abs., volume 90, 125185d).
The various methods illustrated in the prior art for the zone refining of gallium have a number of serious drawbacks that reinforce the statements in the literature that gallium is difficult to purify by zone refining. A complex apparatus is generally necessary that in most cases comprises a helix or an annulus of material that is rotated or wherein a heater is rotated. The use of complex apparatus only allows small amounts of gallium to be purified and large scale production would require a plurality of such apparatus thereby increasing costs considerably. The containment of the gallium becomes a problem when large amounts of high-purity gallium are to be produced. The expansion of gallium when it solidifies rules out the use of Pyrex or quartz for its containment, while materials that have a degree of flexibility that can accommodate expansion, such as polyvinyl chloride and polyethylene, are usually a source of impurities that contaminate the gallium. Another problem in the refining of gallium is its affinity for oxygen. Although vacuum melting and other means have been used to exclude oxygen, the small and complex apparatus of the prior art make it difficult to exclude oxygen. It would, therefore, be advantageous to produce large quantities of high-purity gallium by zone refining with apparatus of simple construction that can accommodate expansions and can be operated such that exposure to oxygen is minimal.